Ionic bombardment cleaning apparatus



y 1 L. A. HOLLAND 2,985,756

IONIC BOMBARDMENT CLEANING APPARATUS Filed Dec. 5, 1958 United StatesPatent IONIC BOMBARDMENT CLEANING APPARATUS Leslie Arthur Holland,Northgate, Crawley, England, as-

signor to Edwards High Vacuum Limited, 'Crawlcy, England, a Britishcompany Filed Dec. 5, 1958, Ser. No. 778,400 Claims priority,application Great Britain Dec. 9, 1957 3 Claims. (Cl. 250-495) Thisinvention relates to ionic bombardment cleaning apparatus, moreparticularly for use in the preliminary cleaning of, for example,optical surfaces to be coated in a vacuum coating chamber. The techniqueof producing mirror surfaces for example, by deposition of a metal filmunder vacuum is well established and it has been recognised for sometime that the glass surface to be coated must be free from anycontaminants in order that a closely adherent film may be produced.

It has been found that the known methods of cleaning by ionicbombardment in which a glass surface to be cleaned is exposed to a glowdischarge at a low prmsure, may, in some circumstances, result in actualcontamination of the surface intended to be cleaned or the formation ofoptically absorbing films upon that surface. Both contamination of thesurface and the formation of optically absorbing films may in factoccur. These highly detrimental efiects occur if the surface is placedfacing a cathode electrode and while the contaminating layers may resultfrom a variety of causes, it isbelieved that they are due todecomposition under electron bombardment from the cathode of residualhydrocarbon molecules temporarily adsorbed on the glass. Again, underspecial conditions intense electron bombardment from a cathode willproduce decomposition of th e oxides in a glass surface."

The object of the present invention is to enable the undoubted efficacyof ionic bombardment to be utilised while reducing or eliminating thecontamination referred to.

According to the present invention, in ionic bombardment cleaningapparatus, a cathode electrode providing bombardment. for the purpose ofproducing an ion'glow discharge is so disposed or arranged that the worksurface is not directly exposed to electrons emitted by or in thevicinity of the cathode electrode, such electrons being acceleratedwithin the cathode dark space and flowing directly towards the worksurface.

In particular forms of apparatus embodying the invention, shieldingmeans are provided between the cathode electrode and the work surface soas to intercept electrons accelerated within the cathode dark space andtravelling directly towards the work surface, such shielding meanspermitting the desired ion glow discharge to be produced Withouthindrance. The invention is applicable to glow discharge cleaningapparatus operated from either AC. or DC. sources.

The provision of shielding means between a cathode and a work surface inaccordance with the invention, is contrary to generally acceptedpractice in ionic cleaning technique and is in fact the reverse of thetechnique advocated in the past for the obtaining of successfulcleaning. In carrying out the invention, the cathode shielding means arepreferably, although not essentially, arranged within the cathode darkspace to avoid wasteful production of positive ions in that region. Ifthe shield is within the cathode dark space, the ionisation equilbriumis disturbed in that region with the result generally of the formshowninFigure 5. Two ring 2,985,756 Patented May 23, 1961 that the dischargemay cease. Accordingly, in carrying out the invention, the electrodes,whether AC. or DC. operated are arranged so that there is provided analternative path of sufiicient length for sustained ionisation elsewherein the chamber when the electrodes are partially shielded within thedark space.

Alternative forms of apparatus constructed and arranged to operate inaccordance with the invention will now be described by way of examplewith reference to the accompanying drawings in which:

Figure 1 is an explanatory diagram.

Figure 2 shows in diagrammatic form one way of carrying out theinvention, 7

Figure 3 shows also diagrammatically a slightly developed form of theapparatus for use with alternating'current,

Figure 4 is a plan view of the arrangement shown in Figure 3,

Figure 5 shows diagrammatically an alternative form of apparatus; and

Figure 6 shows diagrammatically a further form of apparatus.

Referring to the drawings, in Figure 1 is shown a com ventional rotaryWork holder 1 with a negative V-shaped electrode 2 of aluminium arrangedfor cleaning purposes and an anode 3, these components being mountedWithin an enclosing chamber of conventional form; but not shown. Workpieces 16 are shown mounted beneath the Work holder 1. If the dischargeis operated long enough a discoloured deposit may form on the work planefor the reasons already discussed. This deposit can be prevented byplacing a shield electrode 4 as shown in Figure 2 bet-ween'the cathode 5and the Work carried by the holder 1, the geometry being arranged so asto permit a discharge to occur from the edges of the cathode which, asshown, is preferably of rod form. If desired, two such rods may be usedarranged with shields as shown in Figures 3 and 4, the rods beingconnected to either side of an alternating high tension source of supply6. The cathodes and work holding assemblies are contained within anenclosing chamber 17 of conventional form, a pipe connection :18 beingprovided for the alternative connection of an evacuating pump ora'source of gas supply. 1 a

In thebook Vacuum Deposition of Thin Films, by L. A. Holland, there isdescribed a ring type electrode which may be used with a planeor'a'spherical holder electrodes 7 and 8 to which AC. is supplied, arearranged within two earthed shields 9 and 10. The outer shield 10prevents the discharge from reaching the wall of the vessel, not shown,when it is made of glass and the inner shield 9 is arranged to preventmaterial from subsequent evaporation from depositing on the electrodesurfaces. The space between the two ring electrodes is less than theoperative cathode dark space so that the glow discharge is made tofollow a long path within the centre of the vessel. This method ofoperation has been well established for some time but in applying thepresent invention the height of the inner electrode shield 9 is suchthat no part of the spherical work holder 11 is in a straight linebetween the top of the ring electrode 8 and the spherical holder.Instead of increasing the height of the inner electrode shield 9, it maybe fitted with an outwardly extending annular flange. [he vapour sourceis shown at 12.

The AC. operated arrangement shown in Figure 5 may be adapted for DC.operation by making one of the ring electrodes 7 or 8 a cathode and theother an anode. Preferably, the ring 8 constitutes the cathode.

Referring now to Figure 6, with the exception of the provision of acircular shield 13, the form of discharge 3 cleaning apparatus shown isknown. The cathode annular plate 14 is fixed opposite the surface 15 tobe cleaned. After prolonged bombardment, apparatus of this form Withoutthe shield 13 may give rise to discoloured surface layers and theprovision of the shield remedies the effect.

While the arrangement shown in Figure 6 is D.C. operated, it may beadapted for A.C. operation by splitting the annular cathode 14 into twoelements each connected to an AC. terminal.

It is here convenient to remark that some cleaning action may occur whena piece of glass is exposed to a cathode electrode at a gas pressure andwith a separating gap such that the edge of the cathode dark space doesnot extend to the glass surface i.e. the glass is in the positive columnor plasma of the glow discharge. Electrons in a plasma usually attaininsufiicient energy to dissociate hydrocarbon compounds whereaselectrons traversing the cathode dark space can attain energies hundredsof times higher and give rise to decomposition. This method isunsatisfactory because, if the pressure should fall then the dark spaceextends and the glass surface immediately becomes exposed to bombardmentby high velocity electrons, which may induce contami nation. Again, apiece of glass of restricted area may be cleaned by resting it on alarge cathode, but the surface of the glass is eroded by sputtering andmay become electrically charged, thereby aiding diifusion of ions to thesurface of the glass. Both effects are undesirable where optical glassesare concerned.

By using a shielded cathode system in accordance with the presentinvention electron induced contamination is avoided, and the glowdischarge may be operated so that the glass is exposed to bombardment(a) in a plasma i.e. bombarded by positive ions, high energy neutralatoms, or molecules and low velocity electrons or (b) at the sides ofthe cathode space where the high velocity electrons traverse pathsparallel to, or away from, the glass surface, whereas due to theirsmaller mean free path in the residual gas the high energy positive ionsand neutral gas molecules are scattered and bombard the glass surface.Both methods (a) and (b) depend on such factors as voltage and currentdensity of the glow discharge, the nature and pressure of the gas andthe dimensions and geometry of the electrode system. A specific systemmay be operated in either state, usually by changing the workingpressure. Thus, raising the applied voltage and lowering the gaspressure will make the dark space expand causing method (a) to change tomethod (b). The latter method is preferable because it produces agreater temperature rise in the glass and thereby prevents easyre-condensation of matter after cleaning.

Apparatus embodying the invention has been successfully used with avariety of gases, such as nitrogen, hydrogen, oxygen, air and oxygen andeffective cleaning has been achieved.

It will be appreciated that the electrode systems described are arrangedto prevent electrons travelling directly from the electrodes to thesurface undergoing cleaning. Further, the intention is to immerse thesurface to be cleaned in a discharge which is rich in positive ions andhigh energy neutral molecules and atoms.

7 When chemically active gases are used then the cleaning atmosphere isrich in active constituents as well as positive ions and high energyneutral particles. In neither case is the glass exposed to bombardmentby high energy electrons. Accordingly other electrode systems may bedesigned in which the cathode is so shielded that a glow discharge maybe produced in the vessel without high velocity electrons travellingdirectly from the vicinity of or from the cathode surface.

Iclaim:

1. Ionic bombardment cleaning apparatus comprising a chamber containingan ionisable gas and enclosing an electron emitting cathode providingbombardment for the purpose of producing an ion glow discharge, a holderfor work to be cleaned by ionic bombardment and shielding means disposedbetween the cathode electrode and the work surface of said holder so asto intercept electrons travelling directly towards said work surface,such shielding means permitting a desired ion glow discharge to beproduced without hindrance.

2. Ionic bombardment apparatus according to claim 1 in which saidshielding means are disposed within the cathode dark space to avoidwasteful production of positive ions in that region.

3. Ionic bombardment apparatus according to claim 1 in which saidshielding means for electrodes between which said ion discharge isproduced intercept any straight line between the electrodes and saidwork surface.

References Cited in the file of this patent UNITED STATES PATENTS2,373,639 Turner Apr. 10, 1945 2,383,469 Colbert et al Aug. 28, 19452,434,930 Johnson Ian. 27, 1948 2,465,713 Dimmick Mar. 29, 19492,642,535 Schroeder June 16, 1953 2,702,863 Koch Feb. 22, 1955 2,887,411Hoppe et al May 19, 1959

